Cisco Nexus 9000 Series Configuration Manual page 153

Configuring Priority Flow Control
dropped on interfaces that match the PFC queue that is not being drained. Beginning with Cisco NX-OS
Release 7.0(3)I4(2), this feature is supported only for Cisco Nexus 9200 Series switches, Cisco Nexus
93108TC-EX, and 93180YC-EX switches, and Cisco Nexus 9508 switches with the X9732C-EX line
cards.
Beginning with Cisco NX-OS Release 7.0(3)I4(5), this feature is supported on Cisco Nexus 9508 switches
with N9K-X9636PQ line cards and Cisco Nexus 3164Q switches.
• The configuration does not support pausing selected streams that are mapped to a particular traffic-class
queue. All flows that are mapped to the class are treated as no-drop. It blocks out scheduling for the
entire queue, which pauses traffic for all the streams in the queue. To achieve lossless service for a
no-drop class, Cisco recommends that you have only the no-drop class traffic on the queue.
• When a no-drop class is classified based on 802.1p CoS x and assigned a internal priority value
(qos-group) of y, Cisco recommends that you use the internal priority value x to classify traffic on 802.1p
CoS only, and not on any other field. The packet priority assigned is x if the classification is not based
on CoS, which results in packets of internal priority x and y to map to the same priority x.
• The PFC feature supports up to three no-drop classes of any maximum transmission unit (MTU) size.
However, there is a limit on the number of PFC-enabled interfaces based on the following factors:
• You can define the upper limit of any MTU in the system using the systemjumbomtu command. The
MTU range is from 1500 to 9216 bytes, and the default is 9216 bytes.
• The interface QoS policy takes precedence over the system policy. PFC priority derivation also happens
in the same order.
• Ensure that you apply the same interface-level QoS policy on all PFC-enabled interfaces for both ingress
and egress.
Caution
• To achieve end-to-end lossless service over the network, Cisco recommends that you enable PFC on
each interface through which the no-drop class traffic flows (Tx/Rx).
• Cisco recommends that you change the PFC configuration when there is no traffic. Otherwise, packets
already in the Memory Management Unit (MMU) of the system might not get the expected treatment.
• Cisco recommends that you use default buffer sizes for no-drop classes or configure different input
queuing policies suitable to 10G and 40G interfaces and the no-drop class MTU size. If the buffer size
is specified through the CLI, it allocates the same buffer size for all ports irrespective of the link speed
and MTU size. Applying the same pause buffer-size on 10G and 40G interfaces is not supported.
• Do not enable WRED on a no-drop class because it results in egress queue drops.
• Dynamic load balancing cannot be enabled for internal links with PFC. You must disable DLB and
enable RTAG7 load-balancing for internal links with the port-channel load-balance internal rtag7
command.
◦ MTU size of the no-drop class
◦ Number of 10G and 40G ports
Irrespective of the PFC configuration, Cisco recommends that you stop traffic before
applying or removing a queuing policy that has strict priority levels at the interface level
or the system level.
Cisco Nexus 9000 Series NX-OS Quality of Service Configuration Guide, Release 7.x
Guidelines and Limitations for Priority Flow Control
139